The present invention pertains to how to produce and to use semiconductor nanocrystallite embedded glass as a tunable laser active medium. Most tunable lasers use liquid solutions of organic dyes as active lasing media. Since the wavelength, bandwidth, or pulse length of the tunable dye laser can be varied within a range using different dyes in combination with proper optical resonantor designs, the dye laser has been a major workhorse in the advancement of laser spectroscopy. However, the dye molecules react photochemically upon irradiation causing material degradation problems. The design which uses a stable region of a thin dye solution film through a jet nozzle as the active lasing host has lessened the photochemical degradation problem. However, the stability of the jet is still of concern, because it can not be ensured. In addition, the maintenance of the liquid dye circulation system and occasional change of dye solutions demands skilled technical support. Therefore, the search for solid state variants to replace the liquid tunable dye lasers is attractive. Organic dye impregnated polymer or glass as replacement for the dye solution has been considered but so far no product has been developed. The problems are mostly in the area of light induced dye chemical changes at the dye solution/host interface upon irradiation. Recently, Ti-Saphire has been introduced as a tunable solid state laser host. It is tunable in dye laser wavelengths. But the price of Ti-Saphire is high and its tunable range is narrow. It is still far from being able to replace the dye laser.
The semiconductor nanocrystallite embedded glass can be categorized as one of the glass/crystallites composites because it contains more than one phase. The semiconductor crystallite embedded glass is photorefractive due to its high .sub..chi..sup.(3) values. The potential application in holographic data storage, and phase conjugation using this glass has been considered. Some of the semiconductor crystallite embedded glass has been used as a sharp cut-off filter in the optical industry for years. The photoluminescence properties of the sharp cut-off filters has been studied and published before. But no attention has been paid to their potential as tunable laser host materials. This invention modifies the microstructure of the semiconductor nanocrystallite embedded glass so that it can be a practical alternative for replacing the liquid dye in tunable lasers.
The semiconductor nanocrystallite embedded glass contains two distinctly different phases. The glassy phase is composed largely of silicate tetrahedra networks which serve as structural support and transparent window for the semiconductor crystalline phase. The crystalline phase is composed of numerous semiconductor single crystals which are referred to as nanocrystallites here. It is the nanocrystallite which serves as the laser active media. The lasing property of the nanocrystallite is dictated by its composition and its microstructure. The composition determines its peak wavelength of flourescence, and the microstructure determines its fluorescence bandwidth. The microstructure of semiconductor nanocrystallite can be characterized by the size distribution, number density, and the spatial distribution within the glass matrices. Due to the isotropic nature of the glass structure, the orientation of the nanocrystallite is random. Usually, compound semiconductor crystals belong to wursite or zincblend structure and are likely to have direct band gaps. Compound semiconductor single crystals have definite band gap energy depending on the relative chemical composition of the compounds and their orientations. If one assumes that each crystallite resembles a spherical particle, the relative composition, x, varies along its axis. The value of x is the smallest at the core and it increases along the axis. The range of variation is small but is critical for the emission efficiency.
Each of the compound semiconductors possesses a characteristic band gap energy. The magnitude of the band gap energy correlates to a specific wavelength edge below which the irradiation will be absorbed. For instance, zinc sulfide selenide (ZnS.sub.x Se.sub.1-x) absorbs any irradiation wavelength shorter than 450 nm. The shifting of the edge can be obtained by varying the value of x. Optically pumped single crystal platlets used as a laser host have been reported at cryogenic temperatures. The high refractive index of the bulk and high absorption coefficient at fluorescent wavelength hinders the further development of the optically pumped single crystal compound semiconductor.
The semiconductor nanocrystallite embedded glass possess a strong absorption of photons with energy greater than the band gap energy of the nanocrystallite. In contrast to the compound semiconductor single crystal, it has a smaller refractive index and a very low absorption in flourescence wavelength. These factors facilitate conditions favorable for laser application. Dispersed nanocrystallite with each single crystal sizes smaller than 20 nm will prevent concentrated absorption and subsequential heating. Consequently, the penetration depth in the glass ceramics increases and so does the pumping efficiency. The absorbed energy generates electron-hole pairs within the semiconductor nanocrystallite. The generated electrons and holes migrate according to the local electrical field, since there is no net external electrical field imposed on the material, the generated electrons and holes migrate in pairs and are subject to random fluctuation of the local environment. Consequently, the electron and hole pairs recombine and annihilate each other at a characteristic rate called the spontaneous recombination rate. At the same time, photons are emited with a wavelength range corresponding to the energy differences between the recombining pairs of electrons and holes. The emitted photons stimulate further electron hole pair recombinations and facilitate the neccessary coherence for lasing. The emission wavelength is usually longer than that of absorption. This prevents resonant absorption at band gap energy and avoids bleaching of the material. The emission spectrum can be characterized using two parameters: first, full width at half maximum (FWHM), and second, the peak wavelength. For instance, ZnS.sub.x Se.sub.1-x (x=0.09) nanocrystallite embedded glass has photoluminescence spectrum of FWHM=50 nm, and peak at 520 nm. The inhomogeneous broadening of the spectrum is due to the random orientation of the crystallites. This is the major factor which contributes to the wavelength tunability of the lasers.
The size distribution, the spatial distribution, and the number density of the semiconductor nanocrystallite within the glass matrices, and the relative constituents composition (i.e., spatial distribution of x) within each semiconductor nanocrystallite are major attributes for tunable laser application using semiconductor nanocrystallite embedded glass as hosts. This invention provides process information for producing optimized semiconductor nanocrystallite embedded glass. In addition, it also provides the preferred geometrical design of the semiconductor nanocrystallite embedded glass for replacing organic dye solution in a tunable dye laser.